The traditional method of cleaning the belt is to spray wash liquor at high or medium pressure on to one or both surfaces of the belt in the lower run and collect the wash liquor in a receptacle. However, certain substances cannot readily be dislodged with a high pressure spray and require a certain residence time in a suitable solvent. In such cases, the filter belt is passed through a vessel containing a suitable solvent so as to soak the belt for a short period. Guidance of the belt through the vessel is achieved by providing additional fixed guide rollers to deflect the belt down into the liquor and subsequently up again to emerge therefrom.
Apart from the cumbersome nature of such an arrangement, it has certain serious disadvantages. The additional guide rollers impose extra frictional drag on the belt, thus reducing its life. Many of the solvents used are either corrosive (e.g. acid) or degreasing (e.g. hydrocarbon solvents) in nature. In either case the choice of suitable bearings for guide rollers which have to be immersed therein are very limited and tend to result in extra friction being applied to the belt. If the alternative course of locating guide roller bearings outside the soaking vessel is adopted it is necessary to have each roller fitted with stuffing boxes or mechanical seals. This means that the soaking vessel has to be manufactured with great precision to provide appropriate mounting surfaces for such seals and bearings.
In any event, it has been found that a simple soaking operation is usually inadequate to remove small crystals which have lodged or grown in the fabric of the cloth particularly since the residence time of any portion of the belt in the solvent is necessarily limited to less than a minute during each circuit of the belt.